A threat launch detection system is a system that detects a weapon being directed at a target, with the target typically containing the threat launch detection system. In response to detecting a weapon directed at the target, which will be referred to as a threat or event throughout the present description, the threat launch detection system typically takes countermeasures to prevent the weapon from impacting the target. For example, an airplane may include a threat launch detection system designed to detect missiles fired at the airplane. When the system detects a missile, the system typically takes appropriate countermeasures in an attempt to prevent the missile from impacting the airplane, such as transmitting a signal to “jam” electronic circuitry in the missile that is guiding the missile towards the target.
A conventional threat launch detection system is illustrated in FIG. 1, which more specifically depicts a block diagram of a directional infrared countermeasures (DIRCM) system 100. The DIRCM system 100 includes a missile warning system 102 that detects the presence of a weapon or threat 104 directed at an airplane or other vehicle (not shown) containing the system. In the example of FIG. 1, the threat 104 is a missile that has been fired at the airplane containing the DIRCM system 100. The missile 104 includes a guidance system (not shown) for sensing infrared energy emitted by the airplane and for directing the missile towards the airplane.
The missile warning system 102 is typically a passive system that includes a sensor array (not shown) in combination with suitable optics (not shown) to provide a relatively wide field of view WFOV for missiles 104. The wide field of view WFOV is the region of space surrounding the system 100 in which missiles 104 can be detected. The sensor array in the missile warning system 102 may be an array of ultraviolet (UV) or infrared (IR) sensors that capture a series of images within the field of view WFOV. Processing circuitry (not shown) in the missile warning system 102 analyzes the captured images to detect a threat and generates a coarse directional determination indicating an arrival angle at which the missile or other threat 104 is approaching the airplane containing the system 100.
The missile warning system 102 provides this determined arrival angle to a system controller 106 which, in response to the determined angle, applies signals to a fine tracking system 108 to position a fine track sensor (not shown) toward the threat 104 at the determined angle. More specifically, this fine track sensor in the system 108 is typically mounted on a gimbal (not shown) that rotates in response to the signals from the system controller 106 to direct the fine track sensor towards the determined angle and thereby toward the approaching missile 104. The fine track sensor has a narrow field of view NVOV that is much smaller than the wide field of view WFOV to allow the fine tracking system 108 to precisely track the missile 104 or other threat positioned within the narrow field of view.
The fine tracking system 108 further includes a jamming laser (not shown) that is also directed towards the missile 104 by the rotating gimbal. Once the gimbal has positioned the fine track sensor and jamming laser towards the missile 104, the jamming laser is turned on and an infrared jamming laser beam from the laser illuminates the approaching threat 104 missile. This infrared laser energy is modulated in such a way that the when the guidance system in the missile 104 senses this energy the guidance system directs the missile away from the airplane. The fine tracking sensor in the fine tracking system 108 senses the position of the missile 104 during this time to accurately illuminate the missile 104 with energy from the jamming laser. This overall operation of the fine tracking sensor and jamming laser in the fine tracking system 108 may be referred to as “tracking” and “jamming” the threat 104.
The alignment of the jamming laser relative to a mounting datum (not shown) to which the laser is mounted and to the fine tracking sensor must be accurate for proper operation of the fine tracking system 108 in tracking and jamming the threat 104, as will be appreciated by those skilled in the art. As a result, part of the installation and configuration or set-up of the DIRCM system 100 includes properly aligning the jamming laser to the mounting datum. This is typically a manual adjust and set procedure. For example, in one approach an installation person makes manual adjustments to mirrors that contained in optics that direct the laser beam. The installation person manually adjusts these mirrors while looking at the output location of the laser beam using a cooled IR camera to thereby properly align the laser beam. This alignment of the jamming laser beam, however, is time consuming and must be periodically repeated because the alignment tends to drift over time. Also, the alignment is a function of environmental conditions that may cause errors during operation of the tracking and jamming operation of the fine tracking system 108.
In another approach, the jamming laser includes a reference laser and this laser is visible by an un-cooled sensor. Control circuitry then automatically moves a mirror to position the reference laser beam in the correct location to thereby properly position the jamming laser beam. There is no guarantee with this approach that the reference laser is properly aligned to the jamming laser beam. Moreover, with this approach lateral movement of the reference laser beam can not be separated from angular movement and thus alignment accuracy of the jamming laser beam is compromised.
Regardless of which one of these prior approaches for proper alignment of the jamming laser beam is utilized, the jamming laser beam generated by the jamming laser tends to some degree or another to change direction over time. This change is an inherent characteristic of lasers and presents a design issue for the DIRCM system 100 in that the system must have a beam divergence adequate to accommodate changes in direction or angle of the laser beam. Alternatively, the laser must be designed and manufactured in such a way as to ensure that changes in the direction or angle of the jamming laser beam are negligible.
FIG. 2 is a graph illustrating typical shifts in beam position of a 1 milliradian radius laser beam as the beam shifts during ten minutes of operation of the laser generating the beam. The position of the laser beam in a first direction designated the X direction and in a second direction designated the Y direction are shown over seven short intervals T1-T7. During each of these time intervals T1-T7, the top line indicates shifting of the laser beam in the X direction and the bottom line indicates shifting of the beam in the Y direction. Over the total ten minute (600 seconds) time period covered by the graph, the centroid of the laser beam shifts over 2 milliradians in the Y direction as indicated by a position of the beam at the start of interval T1 of approximately 7.3 milliradians and a position of the beam at the end of interval T7 of approximately 5.2 milliradians. If the laser beam was expanded to accommodate this shift, and still given a 1 milliradian error budget to the rest of the system, 90% of the beam energy would be wasted, as will be appreciated by those skilled in the art.
Current approaches that mechanically monitor and adjust the angle of the laser beam have several potential issues. The first is that sensors sensitive laser energy at around 4 microns need to be cooled. Second, mirror actuators are susceptible to vibration and shock and have limited frequency responses for adjusting the position of the laser beam. A third potential issue is that most control loops for controlling such mirrors and actuators are digital control loop and thus have the accompanying issue of aliasing with the movement and firing frequencies of the laser.
There is a need for improved methods and systems for aligning the position of a laser beam and maintaining this alignment over time in countermeasures systems.